There appears to be a --width and --height option for the kwin_wayland command (and they default to the dimensions you mention):

Code:

kwin_wayland --help
Usage: kwin_wayland [options] [/path/to/application...]
KDE window manager
Options:
-v, --version Displays version information.
-h, --help Displays this help.
--lock Disable configuration options
--crashes <n> Indicate that KWin has recently
crashed n times
--author Show author information.
--license Show licence information.
--desktopfile <file name> The base file name of the desktop
entry for this application.
--xwayland Start a rootless Xwayland server.
-s, --socket <socket> Name of the Wayland socket to listen
on. If not set "wayland-0" is used.
--windowed Use a nested compositor in windowed
mode.
--x11-display <display> The X11 Display to use in windowed
mode on platform X11.
--wayland-display <display> The Wayland Display to use in
windowed mode on platform Wayland.
--framebuffer Render to framebuffer.
--fb-device <fbdev> The framebuffer device to render to.
--virtual Render to a virtual framebuffer.
--width <width> The width for windowed mode. Default
width is 1024.
--height <height> The height for windowed mode. Default
height is 768.
--output-count <height> The number of windows to open as
outputs in windowed mode. Default
value is 1
--libinput Enable libinput support for input
events processing. Note: never use in
a nested session.
--drm Render through drm node.
--inputmethod <path/to/imserver> Input method that KWin starts.
--list-backends List all available backends and quit.
--lockscreen Starts the session in locked mode.
--exit-with-session </path/to/session> Exit after the session application,
which is started by KWin, closed.
--no-kactivities Disable KActivities integration.
Arguments:
applications Applications to start once Wayland
and Xwayland server are started